Already for long times various product surfaces have been connected by means of glues. Typical surfaces which are joined together at industrial scale by means of glues include paper and cardboard surfaces. To this end glue paste compositions prepared from starch are widely used because of the ease of their preparation, their technical properties and their availability at economically acceptable costs. Starches from various sources have already been used. Glue paste compositions are commonly prepared from corn starch, wheat starch, potato starch or tapioca starch. The term corn hereinafter always refers to maize.
A particular kind of cardboard which is widely used for various applications is laminated corrugated board, consisting of a cardboard core material having permanent corrugations with an adherent liner on one or on both sides. The liner is commonly a sheet of flat board or paper. The core corrugated material is joined to the adherent liner(s) by means of a glue that is applied to the tops of the corrugates on the upper side and/or the underside of the core material, followed by bringing these tops in contact with the liner(s) and applying pressure and heat.
The manufacture of laminated corrugated board is particularly demanding for the glue because the glue has to be easy to process and has to ensure that during the manufacturing of the laminate the surfaces of the corrugated core and the liner are rapidly glued together with a good initial bond, that the final connection presents good adhesive strength and that possible deformation of the liner caused by the glueing process is minimal.
For the manufacture of laminated corrugated board, mainly glue paste compositions based either on corn starch or on wheat starch are used because these glues ensure good adhesive strength and the source materials are amply available at economically acceptable costs.
Conventionally these glue paste compositions are prepared according to the Stein-Hall method.
Typical for the Stein-Hall method is that the glue composition contains two different parts. The difference results from the fact that the glue composition is composed of a carrier paste (also termed primary paste) and a main paste (also termed secondary paste) the preparations of which are significantly different from each other, as follows from the description below.
The carrier paste is conventionally prepared by mixing the starch in powder form in water warmed up at about 30 to 50° C., with subsequent addition under stirring of a certain amount of sodium hydroxide usually in the form of a concentrated aqueous solution. The mixture is kept for a while at about 30 to 50° C. under vigorous stirring with shearing till a stable viscosity of the mixture is obtained (typically about 15 to 30 minutes), thus forming the carrier paste. The concentration of the starch in the mixture typically ranges from 6 to 16 wt % and the weight ratio sodium hydroxide: starch is commonly ranging from 1:10 to 1:4.
During the preparation process, the starch in the carrier paste becomes fully gellified. Said gellified starch can absorb/bind up to about 20 gram water per gram starch, which is much more than non-gellified starch can do. Non-gellified starch can absorb/bind only about 1 gram water per gram starch.
The main paste is conventionally prepared by stirring the starch in powder form into water at or below room temperature and adding under stirring a certain amount of borax to the mixture. The mixture is stirred for about 10 to 30 minutes, thus yielding the main paste. The concentration of the starch in the mixture ranges from about 18 to 35 wt % and the weight ratio starch: borax commonly ranges from 50:1 to 100:1. Because this mixture is much more concentrated in starch, is containing much less alkali base (borax) based on the starch content than the carrier paste, and is prepared at about or below room temperature, the starch of the main paste does not undergo gellification. As a result thereof, the starch of the main paste is non-gellified starch that can absorb/bind only a small amount of water, about 1 gram water per gram starch.
The glue paste composition is then obtained by mixing the carrier paste with the main paste at a selected weight ratio main paste:carrier paste, which typically ranges from 1:1 to 5:1.
There exist some variants of the glue preparation according to the Stein-Hall method. Following one variant, typically used at industrial scale, the preparation of the glue paste composition is carried out in one reactor (one pot process), involving the subsequent steps of loading the required amount of water, warming it up to about 30 to 50° C., adding the required amount of starch for the carrier paste in powder form under vigorous stirring, adding the appropriate amount of caustic soda solution and stirring the mixture with shearing at about 30 to 50° C. for about 15 to 30 minutes. Accordingly, water at or below room temperature is added under stirring in the required amount for the main paste, followed by the required amount of starch for the main paste in powder form. Then the appropriate amount of borax is added and the mixture is stirred for about 10 to 30 minutes, typically for about 15 minutes, at ambient temperature, thus forming the desired glue paste composition.
As a result of the addition of the amount of water at or below room temperature required for forming the main paste, the temperature of the total mixture and the concentration of the caustic soda in the total mixture are reduced to such a level that the starch added to form the main paste does not become gellified at all or becomes gellified to only a non-significant extent, typically less than 1 wt %, which is considered herein as essentially non-gellified.
According to another variant, also used at industrial scale, the glue paste is prepared by an in-line manufacturing process.
Some glue paste compositions for the manufacture of laminated corrugated board have been disclosed which are based on certain combinations of starches from different plant sources, namely in JP 9-235529 and JP 2001-164213.
JP 9-235529 (Rengo Co Ltd) describes glue pastes which are a mixture of a paste from corn starch and a paste from tapioca starch at a solid content ratio corn starch: tapioca starch of 10:90 to 70:30. The pastes are prepared separately from corn starch and tapioca starch, respectively, water, an aqueous solution of caustic soda and boric acid. In these pastes the weight ratio total water to total starch is 3.00, the total starch concentration is 24.7 wt % , the caustic soda concentration (calculated on the paste) is 0.60 wt % and the content of borax (calculated on the starch content) is 2.00 wt %.
JP 2001-164213 (Tokan Kogyo Co Ltd) describes glue paste compositions for laminating corrugated board prepared from of a mixture of corn starch and rice starch. According to the manufacturing process described by JP 2001-164213, which is based on the Stein-Hall method, a carrier paste and a main paste are prepared separately and subsequently both pastes are mixed to form the glue paste composition. Table 5 of JP 2001-164213 specifically discloses the preparation of a glue paste composition wherein (i) the carrier paste contains a weight ratio corn starch: rice starch of 1:1, a total starch content of 8.6 wt % and a weight ratio total water: total starch of 10.4:1, and (ii) the main paste contains a weight ratio corn starch: rice starch of 4:1, a total starch content of 31.76 wt % and a weight ratio total water: total starch of 2.13:1. Various glue paste compositions are obtained by mixing the carrier paste and the main paste in different weight ratio's.
JP 2001-164213 furthermore stipulates that in order to have the glueing paste presenting good glueing properties, the rice starch has to be present in the carrier starch paste as well as in the main starch paste, and the rice starch has to be present in the glueing paste (being the mixture of the carrier paste and the main paste) in a total concentration ranging from 5 to 30 wt % of the solid moiety of the glue paste.
JP 2001-164213 also discloses as a further characteristic of the invention that the start temperature of the gellification of the glue paste is significantly reduced compared to conventional starch-based glue pastes. As a result thereof the initial glueing is improved and the glueing during the laminating process can be carried out at lower temperatures than the ones used for conventional starch-based glue compositions. JP 2001-164213 relates these properties to the presence in the carrier paste and in the main paste of rice starch which is known to present a lower temperature of gellification and a higher rate of moisture evaporation than starch from corn, wheat, potato and tapioca.
JP 2001-164213 discloses in paragraph 0023 that the presence of rice starch in the carrier paste is of particular importance because it is considered as an important factor to obtain good initial glueing power at reduced glueing temperatures, whereas the effect of rice starch in the main paste appears to be not very large.
The quantity of glueing paste according to JP 2001-164213 that has to be applied to obtain good glueing properties can be reduced by 20% compared to conventional starch-based glue compositions, namely from conventionally about 10 g/m2 down to 8 g/m2.
Furthermore, said reduction in temperature during the initial phase of the laminating process and in amount of glue paste that has to be applied, results in less deformation of the board or paper sheet that is lining the corrugated board core.
Depending from the variety of the rice and the manufacturing process of the rice starch, several types of rice starch are produced, which significantly differ in composition. However, JP 2001-164213 is silent about the type of rice starch used as well as about the effect of the type of rice starch on the physico-chemical properties of the glue paste composition and on its glueing properties.
Although various glue compositions have been disclosed so far and several glue paste compositions for the manufacture of laminated corrugated board are commercially available, these compositions commonly present, apart from desirable properties, mostly one or more disadvantages, such as for example, poor viscosity stability of the glue paste, insufficient glueing strength when applied in small amounts, need of high temperatures to obtain good initial glueing, high water content which leads to high energy requirements (high temperature and/or long heating periods) for evaporating the water during the laminating and glueing step, considerable deformation of the liners, and/or poor manufacturing speed in the laminating/glueing step. Accordingly, industry is continuously in search of new glue compositions for the manufacture of laminated corrugated board, which present improved properties and/or less drawbacks compared to known glue compositions.